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VITAMIN - A:

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Light waves striking these receptors produce chemical changes which in turngives rise to nerve impulse. Rhodopsin / visual purple is a conjuated protein, containing Opsin as its apoprotein and retinene as its prosthetic froup.

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Retinene in rhodopsin is II – cis retinal. (The aldehyde group of II – cis retinal is bpund to £-NH 2 group of Lys. Of Opsin) Even dimlight can break rhodopsin.

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Night blindness: or Nictalopia One of the earliest signals of vit A deficiency Is impaiment of dark adaptation There should be continuous supply of retinol for nromal visual function. Vit A dept depress the resynthesisi of rhydopsin & interferes with the function of rods resulting in night blindness

DARK ADAPTATION:

DARK ADAPTATION More photons are required to excite already illuminated rod, whereas only a few photons are sufficient to excite rod cell in the dark. This property is called Dark adaptation. TOTSTEN WIESEL was awarded Nobel Prize in 1981

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Bright light depletes stores of rhodopsin: When a person shifts suddenly from bright light to a dimly area there is difficulty to see. A few min,later,rhodopsin is resynthesised & vision is improved. This period is called Darkadaptation time.

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This is in vit A deficiency. Defi, of cis- retinal will lead to in darkadaptation time & night blindness.

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5. Preventable blindness: Most common cause of blindness in Indian children below the age of 5. About 40% preventable National blindness control programme give regular vitA supplementation to children L syn

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B.Caratene as Antioxidant: Help to trap free radicals in tissues prevents heart attacks. Retonic acid functions: Role in gene expression & Tissue differentiation Role in GP syn Role in Mps syn – essential for sulfation of MPS in matrix of cartilage & bones.

DIETARY SOURCES:

DIETARY SOURCES ANIMAL: Liver oil,buttermilk, cheese, egg yolk Plant : In the form of Provit carotene,Tomatoes,carrots,green-yellow vegetables,spinach,fruits such as mangoes,Papayas,Corn,Sweet Potatoes. Spirulina: (algae) – Good source of Vit A

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Abs: Vit A precursors are abs. in S.I in the presence of bile salts dietary vit A is in ester form. Hydrolyses by cholesterol esterase into F.A & free Vit A Free Retinol is abs & undergoes reesterification in intestinal epithelial cells. Liver is the only organ where carotena are converted to vit A Storage : stored in liver as retinylester ( retinyl palmitate)

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Transport: Retinol is transported with RBP 95% is stored as ester in itocells of liver released into plasma when required. - 10-20 mg of vit A present per 100 g of liver.

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1Retinalequivalent = 1 ug of Retinol Adult – (3000)IU/ day 5000 Functions: Role in vision Role in Reproduction – no conception takes place unable to carry pregnancy. Role in Epithelialisation Role in Bones& Teeth formation Role in growth.